The invention generally relates to systems and methods for transmitting and receiving wireless communication, and relates in particular to transceivers for wireless telecommunication systems.
Modulation of a carrier with a baseband signal can be achieved using a polar representation of the signal representing the baseband signal as magnitude and phase components. These components are translated onto an RF carrier by functional blocks that modulate the carrier phase and amplitude independently. For reasons of power efficiency combining the component signals is ideally performed at the final stage of the transmitter, which typically is a non-linear power amplifier (PA). When a modulator is implemented in this fashion, time-alignment of the phase and magnitude components is critical for modulated signal integrity.
In addition to modulated signal quality, accurate power control is required by many RF wireless communications standards, such as the GSM/EDGE standard. A transmitter is required to be able to accurately transmit at any of the pre-desired power levels. While it is possible to achieve this goal using open-loop schemes, a closed-loop power control is desirable.
Polar closed-loop systems have been used for many years in transmitter systems. For example Polar-Loop Transmitter, by V. Petrovic & W. Gosling, Electronics Letters, vol. 15, No. 10, (1979) discloses a polar-loop transmitter. U.S. Pat. No. 6,801,784 discloses a continuous closed-loop control system for a wireless transceiver power amplifier that includes modulation injection. See also, An IC for Linearizing RF Power Amplifiers Using Envelope Elimination and Restoration by Su and McFarland, IEEE, JSSC, December 1988, which also discloses systems for closed-loop envelope control.
In a mobile terminal application, typically phase modulation of a carrier is achieved through the use of a phase locked loop. Most common PLL implementations include a phase-locked loop implementation as an offset PLL with a Type 2 loop transfer function (i.e., two poles at zero frequency). Output amplitude (and therefore power) control is typically obtained by using a Type 1 loop.
Polar modulators however, typically have strict time-alignment requirements between the phase and amplitude components. Control of timing-alignment requires control of open-loop gain and bandwidth (3 dB) and phase response of the loops. Maintaining timing alignment over a range of power levels is difficult in certain applications due to non-idealities in the components used.
There is a need therefore, for a more efficient and economic transceiver system for providing a transmitter system in a transceiver system that is linear over a broad range of operating powers and frequencies.